Re-enable FileManagerFileWatcherTest/WatchLocalFile all bots

Use the TestBrowserThreadBundle RunUntilIdle to flush all tasks at the
end of the test, rather than base::Runloop::RunUntilIdle, which caused
flakes on the MSAN/ASAN bots.

Test: unit_tests --gtest_filter=FileManagerFileWatcher*WatchLocalFile
Bug: 813483
Change-Id: I867e5d914074e6c9d389a5f932836cd6cd8a627c
Reviewed-on: https://chromium-review.googlesource.com/1113399Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570264}

chrome/browser/chromeos/file_manager/file_watcher_unittest.cc

@@ -26,6 +26,8 @@ class FileManagerFileWatcherTest : public testing::Test {
       : thread_bundle_(content::TestBrowserThreadBundle::IO_MAINLOOP) {
   }
 
+  void FlushMessageLoopTasks() { thread_bundle_.RunUntilIdle(); };
+
  private:
   content::TestBrowserThreadBundle thread_bundle_;
 };
@@ -106,8 +108,7 @@ TEST_F(FileManagerFileWatcherTest, AddSameExtensionMultipleTimes) {
   ASSERT_EQ(0U, extension_ids.size());
 }
 
-// Disabled for flakiness. crbug.com/813483.
-TEST_F(FileManagerFileWatcherTest, DISABLED_WatchLocalFile) {
+TEST_F(FileManagerFileWatcherTest, WatchLocalFile) {
   const base::FilePath kVirtualPath =
       base::FilePath::FromUTF8Unsafe("foo/bar.txt");
   const char kExtensionId[] = "extension-id";
@@ -140,13 +141,14 @@ TEST_F(FileManagerFileWatcherTest, DISABLED_WatchLocalFile) {
 
   // Create a temporary file in the temporary directory. The file watcher
   // should detect the change in the directory.
-  base::FilePath temp_file_path;
+  base::FilePath temporary_file;
   ASSERT_TRUE(
-      base::CreateTemporaryFileInDir(temp_dir.GetPath(), &temp_file_path));
-  // Wait until the directory change is notified, and also flush the tasks in
-  // the message loop since |change_callback| can be called multiple times.
+      base::CreateTemporaryFileInDir(temp_dir.GetPath(), &temporary_file));
+
+  // Wait until the directory change is notified. Also flush tasks in the
+  // message loop since |change_callback| can be called multiple times.
   change_run_loop.Run();
-  base::RunLoop().RunUntilIdle();
+  FlushMessageLoopTasks();
 
   ASSERT_FALSE(on_change_error);
   ASSERT_EQ(temp_dir.GetPath().value(), changed_path.value());